In radiocommunication systems of this type, digital data encoding an audio signal or, more generally, information of any type, are transmitted by means of an amplitude modulation which takes place in addition to a phase or frequency modulation of the transmitted radiofrequency signal. Thus, the transmitted radiofrequency signal has both a phase or frequency modulation component and an amplitude modulation component. Adding an amplitude modulation component makes it possible in general to improve the bit rate characteristics for a given channel width.
The output stage of the transmitter comprises a radiofrequency power amplifier which, so as to obtain a high power efficiency (this being particularly required in the case of use of a transmitter in portable radiocommunication equipment), must operate within an operating region close to saturation.
Now, as is known, a power amplifier in such an operating region exhibits amplification nonlinearities comprising amplitude nonlinearities and phase non-linearities. In the literature, these nonlinearities are often denoted by amplitude/amplitude conversions (or AM/AM conversions) or amplitude/phase conversions (or AM/PM conversions), respectively. These non-linearities cause distortion of the transmitted signal, which distortion degrades the performance of the transmitter in terms of transmission quality, this loss of quality generally resulting in undesirable broadening of the spectrum.
Various techniques have been proposed for eliminating the effects of the amplification nonlinearities of radiofrequency power amplifiers. These techniques are called radiofrequency power amplifier linearization techniques. In particular, mention may be made of the CLLT (Cartesion Loop Linear Transmitter) technique, the ABP (Adaptive Baseband Predistortion) technique and the EER (Envelope Elimination and Restoration) technique, etc.
The EER technique is very old, since it has been applied since the 50s for amplification of single-sideband (SSB) radiofrequency signals.
The principle of the EER technique is illustrated in FIG. 1, which is a simplified diagram of a radiofrequency signal generator that relies on this technique. The modulation of the radiofrequency signal G output by the generator is decomposed into a phase or frequency modulation component on the one hand, and an amplitude modulation component on the other. These two components are generated in baseband.
In the example shown, a phase modulation component B is delivered, as phase or frequency modulation signal, to the input of phase or frequency modulation means MOD, for example comprising a phase modulator, which transpose it into the radiofrequency range. In a variant of the EER technique, known as the OPLEER (Open Phase Loop EER) technique, the means MOD comprise a phase-locked loop. Such phase modulation means have extremely low broadband noise characteristics because of the high spectral purity that the phase-locked loop can achieve.
The signal E output by the modulation means MOD is a phase-modulated signal of approximately constant amplitude. This signal is then amplified by a radiofrequency power amplifier PA.
An amplitude modulation component C is delivered, as amplitude modulation signal, via circuits (not shown in FIG. 1), to a gain control input of the amplifier PA in order to control the gain of this amplifier. This mechanism allows the amplitude modulation component to be reintroduced into the amplified radiofrequency signal without injecting additional noise. The amplifier PA may be a component having a gain control input or a group of components having a gain control input.
Thus, the amplitude modulation component is super-imposed on the phase modulation component in order to obtain the desired radiofrequency signal G as the output of the amplifier PA, these two components using different paths to reach the output of the amplifier PA.
A radiofrequency transmitter relying on the OPLEER technique is described, for example, in French patent application FR 2 716 589. This transmitter includes AM/AM conversion correction means and AM/PM conversion correction means for the radiofrequency power amplifier PA, in the form of an output signal amplitude control loop and an output signal phase control loop, respectively, which are imbricated.
The diagram of FIG. 2 shows AM/AM conversion correction means for the radiofrequency power amplifier PA, in the case of a generator of the type shown in FIG. 1, which are described in the aforementioned document FR 2 716 589.
These means comprise, for example, an analog control loop for feedback control of the output signal G to the amplitude modulation signal C. The analog control loop includes a comparator amplifier COMP, a first input of which receives the amplitude modulation signal C, a second input of which receives a signal L and the output of which delivers an amplitude control signal F. The latter signal is applied to a gain control input of the amplifier PA. The output of the amplifier COMP is fed back to its second input via an impedance, such as a capacitor C so as to prevent spurious oscillations of the signal F. The signal L is an analog signal representative of the power of the output signal G.
The analog control loop further includes coupling means, such as a radiofrequency coupler 4, for extracting part of the energy of the output signal G and delivering a signal H that is the image of the output signal G.
Finally, it includes a detector DET, the input of which receives the signal H and the output of which delivers the aforementioned signal L. The detector DET allows the amplitude modulation component of the output signal G to be extracted from the signal H by applying rectification and low-pass filtering to the signal H so that the voltage amplitude of the signal L, conventionally expressed in decibel-volts (dBv), is a function of the instantaneous power of the signal H, conventionally expressed in decibels (dBm). The signal L is therefore representative of the amplitude modulation component actually present in the output signal G.
The signal L and the amplitude modulation signal C are very close to each other and differ only by the effect of the AM/AM conversions in the amplifier PA. The signal L is compared to the amplitude modulation signal C by the comparator amplifier COMP, which produces the amplitude control signal F on the basis of their difference.
These AM/AM conversion correction means have the drawback of not controlling the operating point of the amplifier PA. Now, should there be a variation in the supply voltage or in the temperature, for example, the operating point of the amplifier PA may be shifted toward the saturation region of the amplifier PA. Saturation of the amplifier PA generates a distortion of the output signal, without this distortion being detected, or a fortiori corrected.
The graph in FIG. 3a shows the variation in the amplitude of the output signal G in a normal operating case, that is to say when the operating point of the amplifier PA is such that, despite the required amplitude modulation, the entire amplitude variation remains below the saturation point. The graph shown in FIG. 3b gives the corresponding spectrum of the output signal G. As a result of a temperature variation or of a variation in the supply voltage, the operating point may be shifted toward the saturation region of the power amplifier. In certain cases, the analog control loop can no longer provide a sufficient excursion of the amplitude control signal F in order to obtain an output signal G having the required amplitude variation. The amplitude variation shown in the graph of FIG. 3c is then obtained. The corresponding spectrum is shown in the graph of FIG. 3d. As may be seen by comparing these Figures to FIGS. 3a and 3b, the distortion by the clipping of the amplitude then results in spectral broadening of the output signal G.